Methods for obtaining high levels of expression of transfected hybrid human mouse immunoglobulin genes will be developed. These will be combined with rapid methods for construction of hybrid antibody genes to prepare a series of human-compatible immunoglobulins reactive with major T cell surface markers. The methods will be modified to produce immunoglobulins of desired isotype and to switch isotypes of an immunoglobulin gene. Improved methods for localized random mutagenesis of immunoglobulin genes and for selection of genes encoding immunoglobulins with desired properties will be developed and applied to the anti-T cell antibodies.